This invention relates in general to electrophotographic imaging members and more specifically, to imaging members comprising multilayer organic photoconductors utilizing a transport layer in which two or more charge transporting molecules are dispersed in a non-charge transporting binder.
In the art of electrophotography an electrophotographic plate comprising a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging the imaging surface of the photoconductive insulating layer. The plate or photoreceptor is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated area. This electrostatic latent image may then be developed to form a visible image by depositing finely divided toner particles, for example as a dry powder or alternatively suspended in a liquid carrier, on the surface of the photoconductive insulating layer. The resulting visible toner image can be transferred to a suitable receiving member such as paper. This imaging process may be repeated many times with reusable photoconductive insulating layers.
One common type of photoreceptor is a multilayered device that comprises a conductive layer, a blocking layer, an adhesive layer, a charge generating layer, and a charge transport layer. Either the charge generating layer or the charge transport layer may be located adjacent the conductive layer. The charge transport layer can contain an active aromatic diamine small molecule charge transport compound dissolved or molecularly dispersed in a film forming binder. This type of charge transport layer is described, for example in U.S. Pat. No. 4,265,990. Although excellent toner images may be obtained with such multilayered photoreceptors, it has been found that when high concentrations of active aromatic diamine small molecule charge transport compound are dissolved or molecularly dispersed in a film forming binder, the small molecules tend to crystallize with time under conditions such as higher machine operating temperatures, mechanical stress or exposure to chemical vapors. Such crystallization can cause undesirable changes in the electro-optical properties, such as residual potential build-up which can cause cycle-up. Moreover, the range of binders and binder solvent types available for use during coating operations is limited with high concentrations of the small molecules are sought for the charge transport layer.
Another type of charge transport layer has been developed which utilizes a charge transporting polymer. This type of charge transport polymer includes materials such as poly N-vinyl carbazole, polysilylenes, and others including those described in U.S. Pat. Nos. 4,806,443, 4,806,444, 4,818,650, 4,935,487 and 4,956,440. Other charge transporting materials include polymeric arylamine compounds and related polymers described in U.S. Pat. Nos. 4,801,517, 4,806,444, 4,818,650 and 4,806,443 the disclosures of which are incorporated herein by reference in their entirety. Some polymeric charge transporting materials have relatively low charge carrier mobilities. Mechanical properties of these pendant type polymers, such as poly N-vinyl carbazole and polystyryl anthracene, is less than adequate for photoreceptor belt fabrication and operation. Moreover, the cost of charge transporting polymers having high concentrations of charge transporting moieties in the polymer chain can be very costly.
The sensitivity of a layered device depends on several factors: (1) the fraction of the light absorbed, (2) the efficiency of photogeneration within the pigment crystals, (3) the efficiency of injection of photogenerated holes into the transport layer and (4) the distance the injected carrier travels in the transport layer between the exposure and development steps. The fraction of the light absorbed can be maximized by the employment of adequate concentration of pigment in the generator layer and the thickness of the generator layer. The distance the carrier travels in the transports layers depends on the structure of transporting material and the binder and on the concentration of the charge transporting active molecules in the case of transport layers consisting of a dispersion of transport active molecules in a non-transporting inactive binder. However, depending on the structure of the binder and the molecule, crystallization sets in if the concentration of the charge transporting molecules is increased beyond a certain point. Crystallization results in increased residuals and image defects both of which are undesirable. Therefore, the concentration limit of the charge transporting molecules in the transport layer results in a limit to the speed of the xerographic process. If the time between exposure and development is reduced to a value that is lower than the transit time in the charge transport layer of the charge carrier injected from the generator layer, the sensitivity of the device is reduced.
The organic photoreceptor devices currently in use in the reprographic industry employ layered structures which, in general, comprising ground plane, blocking, adhesive, generator and transport layers. The layers are typically either vacuum or solvent coated separately onto an underlying layer or substrate. The surface of the photoreceptor or any of the layers can be constrained due to the interfacial force. The interfacial force S can be related to the external and internal strains .epsilon. applied to the systems, the material Young's modulus E and Poisson's ratio by the following equation: EQU S=(.epsilon..sub.1 -.epsilon..sub.2)/(1/C.sub.1 L.sub.1 +1/C.sub.2 L.sub.2 +L.sub.2 /4.times.(D.sub.1 +D.sub.2)),
where, C.sub.i =E.sub.i /(1-v.sub.i.sup.2) and D.sub.i =C.sub.i L.sub.i.sup.3 /12. This equation has been discussed in detail by Chow, et al. (Polymer Engineering and Science, 17, 436 (1977) and 25, 367 (1985)). It is often critical to eliminate or minimize internal stress build up, which can cause curling of the composite, susceptibility to delamination, and cracking of the brittle layers.
For solvent coated polymeric layers, the internal stress can build up during drying and solvent evaporation processes. The degree of internal stress built up during these processes depends on the interlayer solvent diffusion and the stress relaxation degree of the layers. The latter is a function of the drying temperature, the drying time and the material intrinsic properties. One important parameter that characterizes the material intrinsic properties of stress relaxation during drying is their glass transition temperature, T.sub.g. In general the lower the T.sub.g of the material, the more easily the material can be relaxed within a confined drying temperature and time.
In most organic photoreceptor devices, the internal stress builds up during transport layer coating and drying processes. This is due to the much larger thickness of the transport layer (about 20 micrometers) as compared to the other layers (less than about one micrometers) and due to the brittleness of the generator, adhesive and blocking layers. Therefore, curling, cracking and delamination are often observed on these photoreceptor devices after the transport layer fabrication. It would be advantageous to reduce such internal stress by using a transport layer formula of lower T.sub.g.